Cascode circuits have been used to buffer or isolate a first transistor from voltage variation by series connecting it with a second transistor. By such buffering, the performance of the first or protected transistor is improved. As used in current mirrors, cascoding tends to reduce the variation of current with applied voltage. Cascoding can also be used in amplifiers to decrease the Miller multiplication of the capacitance between the amplifier output and input.
Conventional current mirrors provide an output current proportional to, and often substantially equal to, an input or reference current. By separating the output current from the reference current on different branches or sides of the current mirror, the output current is available to drive high impedance loads. U.S. Pat. No. 5,311,115, issued May 10, 1994 to Archer, describes a variety of current mirrors and their operation.
While a variety of approaches have been taken, many suffer some drawback, such as low output impedance, high reference side voltage drop, need for depletion devices, temperature sensitivity, troublesome leakage currents, second-order effects, etc.
There remains a need for alternative cascode circuits for use in current mirrors and amplifiers.